Protective alumina film and production method thereof

ABSTRACT

Provided is a protective alumina film mainly containing alumina in the α-crystal structure and fine crystal grains in which one or more regions containing additionally an element other than aluminum formed along the planes in the direction almost perpendicular to the thickness direction of the protective film are present intermittently in the thickness direction inside the protective film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/554,601, filed Oct. 27, 2005 which is the U.S. national stage ofInternational Application No. PCT/JP04/03493, filed Mar. 16, 2004, thedisclosures of which are incorporated herein by reference in theirentireties. This application claims priority to Japanese PatentApplication JP2003125518, filed Apr. 30, 2003, the disclosures of whichare incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a protective film mainly containingalumina in the α-crystal structure (hereinafter, referred to simply as“α-alumina”) that is coated on wear-resistant products such as cuttingtools, sliding parts, dies and the like, and a production methodthereof, and in particular to a protective alumina film containing finecrystal grains of α-alumina and a production method of forming theprotective alumina film on a base material or an undercoat film under alow-temperature condition without damaging the properties thereof.

Although the protective alumina film obtained by the present inventioncan be applied to the various applications described above, theinvention will be described hereinafter, applying it mainly to a cuttingtool as a typical example.

BACKGROUND OF THE INVENTION

Generally, base material, for example of high-speed steel or cementedcarbide, having a hard film such as of titanium nitride or titaniumaluminum nitride formed on the surface thereof by physical vapordeposition method (hereinafter, referred to as PVD method), chemicalvapor deposition method (hereinafter, referred to as CVD method), or thelike have been used as cutting tools and sliding parts, for whichsuperior wear resistance and sliding property are demanded.

In particular, for application as a cutting tool, the hard film thereonshould have high wear resistance and heat resistance (oxidationresistance at high temperature), and thus, titanium aluminum nitride(TiAlN), which is superior in both of these properties, has been usedwidely as a coating material for applications such as cemented carbidetools, the edge of which becomes heated to high temperature duringcutting. The reason for such superior properties of TiAlN is that theTiAlN film is improved in heat resistance by the action of aluminumcontained in the film and exhibits high wear and heat resistanceconsistently up to a high temperature of approximately 800° C. VariousTiAlN compounds different in the composition ratio of Ti to Al have beenused as the TiAlN's, but most of them have a composition at a Ti:Alatomic ratio in the range of 50:50 to 25:75, in which the TiAlN issuperior in both properties.

Meanwhile, the edge, for example, of a cutting tool becomes heatedoccasionally to a temperature of 1,000° C. or higher during cutting.Under such circumstance, because the TiAlN film alone is notsatisfactory as it is for ensuring sufficient heat resistance, analumina layer is formed additionally on the TiAlN film for ensuring heatresistance, as disclosed, for example, in U.S. Pat. No. 5,879,823.

Alumina has various crystal structures depending on temperature, but anyof the crystal structures except α-crystal structure is in the thermallymetastable state. However, as in the case of a cutting tool, when theedge temperature during cutting fluctuates in a wide range from roomtemperature to 1,000° C. or higher, the crystal structure of aluminachanges, causing problems such as cracking and delamination of film.However, alumina in the α-crystal structure once formed by CVD at ahigher substrate temperature of 1,000° C. or more retains its thermallystable structure, independently of the temperature thereafter. Thus,coating of an alumina film in the α-crystal structure is considered asan effective means of adding heat resistance to cutting tools andothers.

However as described above, it is necessary to heat the substrate to1,000° C. or higher to form alumina in the α-crystal structure, whichrestricts the kinds of base materials applicable. It is because the basematerial may soften and lose its favorable qualities as a substrate forwear-resistant products depending on its kind, when exposed to a hightemperature of 1,000° C. or higher. In addition, even highheat-resistant base materials, such as cemented carbides, also causeproblems such as deformation, when exposed to such a high temperature.Practical temperature range for use of the hard films, such as TiAlNfilm, formed on a base material as an wear-resisting film is generallyapproximately 800° C. at the highest, and the film may decompose andlead to deterioration in wear resistance when exposed to a hightemperature of 1,000° C. or higher

To overcome such problems, U.S. Pat. No. 5,310,607 reported that acomposite (Al,Cr)₂O₃ crystal having a hardness at the same level as thatof the alumina described above was obtained in a low-temperature rangeof 500° C. or lower. However, if the work material is a materialcontaining iron as the principal component, Cr existing on the surfaceof the composite crystal film often reacts chemically with iron in thework material during cutting, causing enhanced consumption of the filmand reduction of the lifetime.

Alternatively, O. Zywitzki, G. Hoetzsch, et al. reported in “Surf. Coat.Technol.” (86-87, 1996, pp. 640-647) that it was possible to form analuminum oxide film in the α-crystal structure at 750° C. by reactivesputtering by using a pulsed power supply at a high output (of 11 to 17kW). However, it is inevitable to expand the size of the pulsed powersupply for obtaining aluminum oxide in the α-crystal structure by thismethod.

As a method for overcoming such problems, Japanese Unexamined PatentPublication No. 2002-53946 discloses a method of using an oxide film incorundum structure (α-crystal structure) having a lattice parameter of4.779 Å or more and 5.000 Å or less and a film thickness of at least0.005 μm as an underlayer and forming an alumina film in the α-crystalstructure on the underlayer. It was indicated there that the componentfor the oxide film is preferably Cr₂O₃, (Fe, Cr)₂O₃ or (Al,Cr)₂O₃; whenthe component for the oxide film is (Fe, Cr)₂O₃, it is more preferably(Fe_(x), Cr_((1-x)))₂O₃ (wherein, 0≦x≦0.54); and when the component forthe oxide film is (Al,Cr)₂O₃, it is more preferably(Al_(y),Cr_((1-y)))₂O₃ (wherein, 0≦y≦0.90).

The Japanese Unexamined Patent Publication No. 2002-53946 above alsodisclosed that it was effective to form a film of a composite nitride ofAl and one or more elements selected from the group consisting of Ti,Cr, and V as a hard film and additionally as an intermediate layer anoxide film having the corundum structure (α-crystal structure) byoxidizing a film of (Al_(z), Cr_((1-z)))N (wherein, 0≦z≦0.90) and thenform alumina in the α-crystal structure on the oxide film.

As described above, it is possible to form an alumina film almost onlyin the α-crystal structure according to the method described in theJapanese Unexamined Patent Publication No. 2002-53946, but the inventorshave found that the method often causes growth to coarse crystal grainsin the alumina film, from the following experiments:

According to the method described in Japanese Unexamined PatentPublication No. 2002-53946, a CrN film was formed on cemented carbide,the surface thereof was oxidized, and an alumina film was formed on theoxidized CrN film surface, and the alumina film obtained was observed.

More specifically, a sample 2 in which a CrN film was formed on acemented carbide base material by ion plating method (AIP method) wasprepared, and the sample was connected to the planetary revolving jig 4and heated to 750° C. with the heaters 5 after the chamber 1 wasevacuated to almost vacuum state in the device shown in FIG. 1. When thesample 2 is heated to a predetermined temperature, an oxygen gas wasintroduced into the chamber 1 at a flow rate of 300 sccm to a pressureof approximately 0.75 Pa, and the sample 2 was oxidized while heated for20 minutes.

Then, an alumina film was formed on the undercoat film after oxidation.The alumina film was formed by the reactive sputtering method, i.e., byheating the base material to a temperature similar to that in theoxidation step (750° C.) in an argon and oxygen environment whileapplying a pulsed DC power of about 2.5 kW respectively to the twosputtering cathodes 6 each with an aluminum target shown in FIG. 1. Theprotective alumina film was formed at a discharge condition in aso-called transition mode, while controlling the discharge voltage andthe flow rate ratio of argon/oxygen by using plasma emissionspectroscopy. A protective alumina film having a thickness ofapproximately 2 μm was formed in this manner.

The crystal structure of the alumina film was identified by analyzingthe surface of the alumina film obtained by using a thin-film X-raydiffractometer. As a result, only diffraction peaks indicating α-aluminawere observed as the diffraction peaks indicating alumina, confirmingthat the alumina film obtained by the method was an alumina filmpredominantly containing α-alumina.

Then, the alumina film was observed under a SEM (scanning electronmicroscope). The surface micrograph is shown in FIG. 2. FIG. 2 revealsthat the alumina film obtained by the method is superior incrystallinity and has crystal grains definitely distinguishable, becauseof drastic crystal growth even at a film thickness of approximately 2μm. The alumina film in such a surface state seems to have an increasedsurface roughness.

Then, the cross section of the alumina film was observed under a TEM(transmission electron microscope), and the film was subjected to an EDSanalysis. The result is shown in FIG. 3.

FIG. 3 reveals that the film consists of three layers: from the basematerial, a CrN film, a chromium oxide (Cr₂O₃) layer having a thicknessof 30 to 40 nm obtained by oxidation of the CrN film surface, and analumina film. The results by electron diffraction pattern confirmed thatthe alumina and chromium oxides are both in the corundum structure.

The micrograph of FIG. 3 shows that the alumina crystal grainsconstituting the alumina film are grown larger as they become closer tothe film surface. Increase in the surface roughness of alumina film dueto growth to coarse crystal grains may cause problems, depending onapplications. For example, when the film is applied to a cutting tool,the work material may be more likely adhered to the alumina filmsurface.

The present invention was completed under the circumstances above, andan object thereof is to provide a protective alumina film containingfine crystal grains of alumina in the α-crystal structure, and a methodof producing the protective alumina film.

SUMMARY OF THE INVENTION

The protective alumina film according to the present invention is aprotective film mainly containing alumina in the α-crystal structure,characterized in that one or more regions containing additionally anelement other than aluminum formed along the planes in the directionalmost perpendicular to the thickness direction of the protective filmare present intermittently in the thickness direction inside theprotective film (hereinafter, referred to as the “first protectivealumina films”). The element other than aluminum is preferably a metalelement capable of forming its oxide, and use of Cr and/or Fe as themetal element is particularly preferable.

The first protective alumina film preferably has the mixed regionsseparated from each other at a distance of 0.5 μm or less, and the totalthickness of the mixed regions is preferably 10% or less with respect tothe thickness of the protective film. In addition, the content of theelement other than aluminum is preferably 2 atom % or less in the entireprotective film.

The present invention also specifies a protective alumina film inanother configuration, and the protective alumina film is a protectivefilm mainly containing alumina in the α-crystal structure, characterizedin that alumina layers mainly in the α-crystal structure are laminatedalternately with layers of an oxide in the corundum structure (excludingalumina, the same shall apply hereinafter) or layers of the oxide in thecorundum structure mixed with the metal of the oxide (hereinafter,referred to as the “second protective alumina film”).

The oxide in the corundum structure is preferably Cr₂O₃, Fe₂O₃ or amutual solid solution of two or more oxides selected from the groupconsisting of Cr₂O₃, Fe₂O₃ and Al₂O₃; the thickness of the oxide layerin the corundum structure or the mixed layer of the oxide in thecorundum structure and the metal of the oxide is preferably 2 nm ormore; and the total thickness of the oxide layers in the corundumstructure or the mixed layers of the oxide in the corundum structure andthe metal of the oxide is preferably 10% or less with respect to thethickness of the protective film. In addition, the thickness of thealumina layer mainly in the α-crystal structure is preferably 0.5 μm orless.

The present invention specifies a protective alumina film in yet anotherconfiguration, and the protective alumina film is a protective filmmainly containing alumina in the α-crystal structure, characterized bybeing a laminate film of alumina layers mainly in the α-crystalstructure substantially different in crystal nucleus (hereinafter,referred to as the “third protective alumina film”). The thickness ofthe alumina layer mainly in the α-crystal structure is preferably 0.5 μmor less.

The term “substantially” means that the third protective alumina filminclude the case when a newly laminated alumina layers mainly in theα-crystal structure is completely different in crystal nucleus from theundercoat alumina layer mainly in the α-crystal structure as well as thecase when a newly laminated alumina layer mainly in the α-crystalstructure contains a small amount of regions wherein the crystal nucleiof the undercoat alumina layer mainly in the α-crystal structure aregrown as they are.

The present invention specifies a method of producing the protectivealumina films, and the method of forming the first or second protectivealumina film (forming a mutual solid solution of an oxide excludingalumina in the corundum structure and Al₂O₃ as the oxide layer in thecorundum structure) on a base material (including a base material havingan undercoat film previously formed) is characterized by depositing anelement other than aluminum intermittently while forming alumina layersmainly in the α-crystal structure during formation of the protectivealumina film.

The method of forming the second protective alumina film (forming anoxide excluding alumina in the corundum structure as the oxide layer inthe corundum structure) on a base material (including a base materialhaving an undercoat film previously formed) is characterized by formingalumina layers mainly in the α-crystal structures and oxide layers inthe corundum structure (excluding alumina) alternately during formationof the alumina film.

The method of forming the third protective alumina film on a basematerial (including a base material having an undercoat film previouslyformed) is characterized by forming alumina layers mainly in theα-crystal structure intermittently during formation of the alumina film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory view (top view) illustrating a deviceused in working the present invention.

FIG. 2 is an SEM observation micrograph (magnification: 8,000 times)showing the surface of an alumina film obtained by a conventionalmethod.

FIG. 3 is a TEM observation micrograph (magnification: 20,000 times)showing the cross section of an alumina film obtained by a conventionalmethod.

FIG. 4 is a schematic drawing of the crystal grains in FIG. 3.

FIG. 5 is a schematic cross-section explanatory drawing showing a statein which alumina layers of fine crystal grains are laminated.

FIG. 6A is a schematic cross-section explanatory drawing showing theconfiguration of a first protective alumina film, while FIG. 6B is agraph showing the composition of the components of the protective film.

FIG. 7A is a schematic cross-section explanatory drawing showing theconfiguration of a second protective alumina film, while FIG. 7B is agraph showing the composition of the components of the protective film.

BEST MODE FOR CARRYING OUT THE INVENTION

Under the circumstances above, the inventors studied the methods offorming a protective alumina film mainly containing fine alumina in theα-crystal structure on a base material such as cemented carbide or on abase material having a hard film formed as an undercoat film, forexample, of TiAlN, at relatively lower temperature.

FIG. 4 is a schematic view illustrating the crystal grains of α-aluminadrawn with the seemingly grain boundary lines extracted from the TEMmicrograph shown in FIG. 3.

FIG. 4 above shows that crystal grains of α-alumina 13 grow larger asthe film is thickened and the width of the crystal grains reaches up toapproximately 0.5 μm in the region close to the alumina film surface,which in turn indicates that the alumina film has a surface irregularityof as large as approximately 0.5 μm.

However, observation of FIG. 4 in detail reveals that relatively smalleralumina crystal grains are formed on the chromium oxide layer 12 in theinitial phase of the crystal grain growth, and thus, the alumina crystalgrains in the alumina film will remain fine in size and the alumina filmsurface smooth, if the alumina film deposition is terminated when thesmaller alumina crystal grains are formed.

FIG. 4 above shows that crystal grains remain small when the thicknessof alumina layer is 0.5 μm or less, and it is possible to suppress thegrowth to coarse crystal grains by restricting the thickness of aluminalayer, more preferably, to 0.2 μm or less.

That is, as shown schematically in FIG. 5, it would be possible toobtain a protective alumina film 15 consisting of several layers offine-crystal grain α-alumina layers 16, by forming an alumina monolayeras a fine-crystal grain thin film as described above followed byrepeating to form alumina layers after bringing the crystal grain growthback to the initial phase.

Zywitzki, Takamura, et al. observed the cross section of an alumina filmmainly in the α-crystal structure obtained by the PVD method and foundthat there was alumina in the γ crystal structure (hereinafter, referredto simply as “γ-alumina”) observed in the initial fine-crystal regionformed close to the interface with the base material, and reported fromthe observed results that the α-alumina crystals were grown from theγ-alumina crystals (e.g., Surface and Coatings Technology, 1997, pp. 303to 308, ibid., 2001, pp. 260 to 264). Accordingly, it was consideredthat contamination of γ-alumina was inevitable in the fine-aluminacrystal grain region formed in the initial phase of growth whenα-alumina is formed by the PVD method.

However, when the inventors analyzed the alumina in the region close tothe interface with the Cr₂O₃ layer shown in FIG. 3 above by using athin-film X-ray diffractometer, there were found only diffraction peaksindicating α-alumina and no peaks indicating γ-alumina in any area closeto the interface.

That is, it was confirmed that it was possible to form a film mainlycontaining α-alumina even in the initial phase of crystal grain growthif it was formed under a suitable condition.

Based on these findings, the inventors conducted studies on typicalexamples of such a protective alumina film, for the purpose of providinga protective alumina film of fine α-alumina that is formed in theinitial phase of crystal grain growth, and have found the protectivealumina films of embodiments 1 to 3 described above. Hereinafter, theconfiguration of each protective alumina film will be described indetail.

<First Protective Alumina Film>

The first example of the protective alumina film of fine α-alumina is aprotective alumina film in the configuration in which one or moreregions containing an additional element other than aluminum (mixedregions) formed along the plane in the direction almost perpendicular tothe thickness direction of the protective film are presentintermittently in the thickness direction inside the protective film.Hereinafter, the region almost of α-alumina between the mixed regionswill be referred to as an “alumina layer”.

The protective films include, for example, a film having multiple mixedregions 14 formed between alumina layers 16 shown in FIG. 5, and a moreschematic view of the protective film is illustrated, for example, inFIG. 6A. FIG. 6A shows that the protective alumina film 15 formed on anoxidation layer 22 obtained by oxidation of a base material or anundercoat film 21 has four mixed regions between alumina layers 23, andthe protective film 15 has the composition of the components shown inFIG. 6B.

Thus, it would be possible to disturb growth of crystal grainsdeliberately by adding an element other than aluminum during theα-alumina crystal-growing process and generate a point to initiategrowth of new crystal nuclei.

Although the following description is not intended to specify theconfiguration of the first protective film in detail, for obtainingfiner α-alumina, the mixed regions and the like in the protective filmare preferably formed as follows:

The thickness of the layer (alumina layer) between the mixed regions ispreferably 0.5 μm or less. It is for the purpose of suppressing growthto coarse crystal grains as described above, and the thickness is morepreferably 0.2 μm or less.

The element other than aluminum is not particularly limited, butpreferably a metal element capable of forming its oxide. From theviewpoint of forming new α-alumina on the mixed region, it is alsofavorable to use Cr and/or Fe that form oxides having the same crystalstructure as α-alumina (oxide in the corundum structure) as the metalelement.

The total thickness of the mixed regions is preferably 10% or less withrespect to the thickness of the protective film.

It is not necessary to set an upper limit for the thickness of the mixedregions in particular for the purpose of inhibiting the alumina crystalgrowth and forming new nuclei for crystal growth, but the protectivefilm should be made mainly of α-alumina for obtaining thehigh-temperature stability inherent to α-alumina, and thus, the totalthickness of the mixed regions in the protective film is 10% or lesswith respect to the thickness of the protective film for that purpose.The total thickness of the mixed regions is more preferably 5% or less,still more preferably 2% or less, with respect to the thickness of theprotective film.

The content of the element other than aluminum in the mixed region is 2atom % or less in the entire protective film. It is because addition ofthe element other than aluminum to the protective alumina film in agreater amount may result in adverse effects such as diffusion at hightemperature. The content of the element other than aluminum in the mixedregion is more preferably 1 atom % or less and still more preferably 0.5atom % or less.

<Second Protective Alumina Film>

Another example of the protective alumina film having finer crystalgrains is a protective film in which alumina layers mainly in theα-crystal structure are laminated alternately with layers of an oxide inthe corundum structure (excluding alumina, the same shall applyhereinafter) or layers of the oxide in the corundum structure mixed withthe metal of the oxide (hereinafter, referred to simply as “mixedlayers”). Thus, it is possible to form α-alumina layers having finecrystal grains and suppress generation of γ-alumina, by laminating alayer containing an oxide in the same crystal structure as alumina inthe α-crystal structure alternately with an α-alumina layer.

The protective films include, for example, those having multiple oxidelayers or mixed layers 14 in the corundum structure respectively betweenthe alumina layers 16, as shown in FIG. 5. FIG. 7 is a schematic viewillustrating the protective layer when a Cr₂O₃ layer is formed as theoxide layer in the corundum structure.

The protective film 15 shown in FIG. 7 is a film obtained by repeatingthe steps of forming a Cr₂O₃ layer 25 in the corundum structure on anα-alumina layer 23 during crystal growth and another α-alumina layer 23thereon, and the protective film has the composition of the componentsshown in FIG. 7B.

The oxide in the corundum structure is favorably Cr₂O₃, Fe₂O₃, or amutual solid solution of (Cr₂O₃ and/or Fe₂O₃) and Al₂O₃ [e.g.,(Al_(1-x)Cr_(x))₂O₃ or (Al_(1-x)Fe_(x))₂O₃] and is not limited to theCr₂O₃ described above.

The thickness of the alumina layer is favorably 0.5 μm or less. It isfor the purpose of suppressing generation of coarse crystal grainssimilarly to the first protective film, and the thickness is morepreferably 0.2 μm or less.

The oxide layer or the mixed layer in the corundum structure ispreferably a single layer that has a thickness allowing independentrecognition of the crystals, i.e., a thickness of 2 nm or more.

The total thickness of the oxide layer or the mixed layers in thecorundum structure is preferably 10% or less with respect to thethickness of the protective film. It is because a higher rate of theoxide layers containing an element other than aluminum in the protectivefilm may lead to adverse effects such as diffusion at high temperature.The total thickness of the oxide or mixed layers in the corundumstructure is more preferably 1% or less.

<Third Protective Alumina Film>

Another example of the protective alumina film having finer crystalgrains is a laminate protective film consisting of alumina layers mainlyin the α-crystal structure (α-alumina layers) that are substantiallydifferent from each other in crystal nucleus. As will be describedbelow, it is possible to obtain the laminate protective alumina filmconsisting of fine-crystal grain α-alumina layers that is only made offine-crystal grain α-alumina layers, by stopping α-alumina depositionbefore growth of alumina crystal nuclei and then repeating deposition ofα-alumina.

As described above, the thickness of the α-alumina layer is preferably0.5 μm or less, more preferably 0.2 μm or less, for obtaining a laminateprotective alumina film consisting of fine-crystal-grain α-aluminalayers.

<Base Material and Undercoat Film>

The protective alumina film according to the present invention is formedon a base material or on a base material having an undercoat filmpreviously formed, but the base material or the undercoat film is notparticularly limited. Examples of the base materials for use includesteel materials such as high-speed steel, cemented carbide, sinteredcermet or cBN (cubic boron nitride), and sintered ceramics.

Alternatively, a film made of one or more compounds of one or moreelements selected from the group consisting of the elements in Groups4a, 5a and 6a in the periodic table, Al, Si, Fe, Cu and Y with one ormore elements of C, N, B, and O, or a mutual solid solution of thesecompounds is preferably used as the undercoat film for deposition of thealumina in the α-crystal structure.

Typical examples of the undercoat films include films of Ti(C,N),Cr(C,N), TiAl(C,N), CrAl(C,N), and TiAlCr(C,N), i.e., of carbides,nitrides, and carbide-nitrides respectively of Ti, Cr, TiAl, CrAl, andTiAlCr; and for example, a single or multilayer film of TiN, TiC, TiCN,TiAlN, CrN, CrAlN, or TiAlCrN can be formed as the hard film on commonlyused cutting tools and others.

In the present invention, the base material or the undercoat film ispreferably used after surface oxidation for deposition of the protectivealumina film, because the alumina layer mainly in the α-crystalstructure is prepared easily.

<Production Method>

[Production Method (1)]

For the purpose of forming the first or second protective alumina film(forming a mutual solid solution of an oxide in the corundum structureexcluding alumina and Al₂O₃ as the oxide in the corundum structure) on abase material (including a base material having an undercoat filmpreviously formed), it is preferable to deposit an element other thanaluminum intermittently while forming alumina layers mainly in theα-crystal structure during formation of the protective alumina film.

A specific example thereof is a method of forming a protective aluminafilm on a base material (including a base material having an undercoatfilm previously formed) by depositing a metal of an element other thanaluminum or the oxide thereof or the like intermittently by using asputtering source containing the metal of an element other than aluminumor the alloy or metal oxide containing the element as the target whileperforming reactive deposition by sputtering an aluminum target in amixed environment of Ar and oxygen.

[Production Method (2)]

An example of the method of forming the second protective alumina film(forming an oxide in the corundum structure excluding alumina as theoxide in the corundum structure) on a base material (including a basematerial having an undercoat film previously formed) is a method offorming alumina layers mainly in the α-crystal structure and oxidelayers in the corundum structure (excluding alumina) or mixed layers ofan oxide in the corundum structure and the metal of the oxidealternately during formation of the alumina film.

A specific example thereof is a method of depositing a protectivealumina film on a base material (including a base material having anundercoat film previously formed) by forming an alumina film bysputtering an aluminum target in a mixed environment of Ar and oxygenand then discontinuing the deposition of the alumina film once; formingan oxide layer in the corundum structure or a mixed layer of an oxide inthe corundum structure and the metal of the oxide by sputtering using atarget containing the metal for the oxide in the corundum structure(including alloy) in a mixed environment of Ar and oxygen; and thenrepeating deposition of the alumina film and the oxide layers in thecorundum structure.

[Production Method (3)]

A specific example of the method of forming the third protective aluminafilm on a base material (including a base material having an undercoatfilm previously formed) demands intermittent deposition of aluminalayers mainly in the α-crystal structure during formation of the aluminafilm. It includes a method of discontinuing deposition of an aluminalayer once, lowering the base material temperature, and then forminganother alumina layer while the base material temperature is raisedagain.

EXAMPLES

Hereinafter, the present invention will be described more specificallywith reference to Examples, but it should be understood that the presentinvention is not restricted by the following Examples and allmodifications that may be made in the scope of the descriptions aboveand below are also included in the technical scope of the presentinvention.

Comparative Example

A sample of cemented carbide base material having a CrN film previouslyformed by the AIP method was made available, and after oxidation of thesample surface, a protective alumina film was deposited thereon. Theoxidation and the deposition of the protective alumina film wereperformed in the vacuum deposition device shown in FIG. 1 (AIP-S40hybrid coater, manufactured by Kobe Steel).

The oxidation is performed specifically as follows: The sample (basematerial) 2 was connected to a planetary revolving jig 4 on therevolving table 3 in chamber 1; after evacuating the chamber 1 almostinto the vacuum state, the sample 2 was heated to 750° C. (base materialtemperature in oxidation step) with the heater 5 placed in the center ofthe chamber 1 and two heaters 5 placed on the inner side wall of thechamber 1. When the temperature of sample 2 reached to a predeterminedtemperature, an oxygen gas was introduced into the chamber 1 at a flowrate of 300 sccm to a pressure of approximately 0.75 Pa, and the samplewas oxidized while heated in the same state for 20 minutes.

The heating, the oxidation, and the alumina deposition described belowwere performed while revolving the table 3 shown in FIG. 1 above androtating the planetary revolving jig 4 (base material-holding pipe)placed thereon.

Then, a protective alumina film was formed on the undercoat film afteroxidation. The protective alumina film was formed by the reactivesputtering method of heating the base material to a temperature similarto that in the oxidation step (750° C.) and applying a pulsed DC powerof about 2.5 kW to the two sputtering cathodes 6 each with an aluminumtarget in a mixed environment of Ar and oxygen. The protective aluminafilm was formed at a discharge condition in a so-called transition-mode,while controlling the discharge voltage and the flow rate ratio ofargon/oxygen by using plasma emission spectroscopy. A protective aluminafilm having a thickness of approximately 2 μm was formed in this manner.The period for forming the protective alumina film was 3 hours.

Inventive Example 1

The following experiment was performed as an Inventive Examples: Aprotective alumina film was formed in a similar manner to theComparative Example above, except that the protective alumina film wasdeposited in the following way after the one of the two aluminum targetson sputtering cathode 6 was replaced with a Cr target.

The protective alumina film was formed by depositing an alumina layerfirst by using only one aluminum target of the sputtering cathode 6 for57 minutes and then applying a voltage of 500 W to the sputteringcathode 6 with a Cr target for 3 minutes while continuing deposition ofthe alumina layer. The steps were repeated four times, and aluminalayers were formed over a period of 1 hour.

A protective alumina film having a total thickness of approximately 2 μmthat mainly contains α-alumina and additionally Cr-containing mixedregions at an interval of approximately 0.35 μm from the base materialsurface was prepared in this manner.

Inventive Example 2

Then, a protective alumina film was formed in a similar manner toInventive Example 1 by oxidizing the same sample, except that the Crtarget used in Inventive Example 1 was replaced with a Ti target.

That is, the protective alumina film was formed by depositing an aluminalayer first by using the sputtering cathode 6 with an aluminum targetfor 57 minutes and then applying a voltage of 500 W to the sputteringcathode 6 of Ti target for 3 minutes while continuing deposition of thealumina layer. Alumina layers were formed over a period of 1 hour, byrepeating the steps four times.

A protective alumina film having a total thickness of approximately 2 μmthat mainly contains α-alumina and additionally Ti-containing mixedregions at an interval of about 0.35 μm from the base material surfacewas prepared in this manner.

Inventive Example 3

In a similar manner to Inventive Example 1, a protective alumina filmwas formed by oxidizing the same sample in a similar manner to theComparative Example above, except that an aluminum target and a Crtarget were connected to the sputtering cathodes 6, as follows:

The protective alumina film was formed by forming an alumina layer firstby using only one sputtering cathode 6 with an aluminum target for 55minutes and then, after discontinuing the deposition of the aluminalayer, applying a voltage of 2.5 kW to the sputtering cathode 6 with aCr target for 5 minutes. Alumina layers were formed over a period of 1hour, by repeating the steps for four times.

A protective alumina film having a total thickness of approximately 2 μmthat mainly contains α-alumina and additionally Cr₂O₃ layers at aninterval of about 0.35 μm from the base material surface was prepared inthis manner.

Inventive Example 4

In this Inventive Example, a protective alumina film was formed in asimilar manner to the Comparative Example above by oxidizing the samesample, except that the protective alumina film was formed while twoaluminum targets were connected to the sputtering cathodes 6 in chamber1, as follows:

In preparing the protective alumina film, deposition of an alumina layerwas stopped at a point of 30 minutes after start by terminatingsputtering and heating by heaters 5. The heaters 5 were turned on after10 minutes, and then sputtering was resumed after minutes from then,while supplying the environmental gas (Ar+O₂) used during sputteringabove at the same flow rate. Alumina layers were formed over a period of30 minutes, by repeating the steps four times.

The protective film obtained is a laminate film having a total thicknessof approximately 2 μm that contains alumina layers substantiallydifferent in crystal nucleus at an interval of approximately 0.35 μmfrom the material surface.

<Thin-Film X-Ray Diffraction Analysis and SEM Observation of theProtective Alumina Film Obtained>

The surface of each protective alumina film thus obtained was analyzedwith a thin-film X-ray diffractometer, and the crystal structure of eachprotective alumina film was identified. In addition, the surface of eachprotective alumina film was observed under a SEM. The results aresummarized in Table 1.

TABLE 1 Surface X-ray diffraction result smoothness Comparativeα-Alumina + Many crystal Example fine α-Cr₂O₃* grains in the order of0.5 to 1 μm Example 1 α-Alumina Smooth Example 2 α-Alumina SmoothExample 3 α-Alumina + Smooth fine α-Cr₂O₃* Example 4 α-Alumina Smooth*In the oxide-containing layer

As apparent from Table 1, in the protective alumina film mainlycontaining α-alumina formed in the Comparative Example, i.e., formed bya conventional method, the alumina crystal grains are larger and coarse.On the contrary, it is obvious that the alumina in each of theprotective alumina films according to the present invention of InventiveExamples 1 to 4 is almost all in the α-crystal structure and in minutecrystal grains and has a smooth surface, due to suppression of the growof crystal grains.

INDUSTRIAL APPLICABILITY

The present invention in the configuration described above provides aprotective alumina film mainly containing α-alumina and aluminafine-crystal grains that can be expected to be potentially superior inwear and heat resistances to those traditionally available, and a methodof producing the same.

1. A protective alumina film comprising alumina in the α-crystalstructure, wherein alumina layers mainly in the α-crystal structure arelaminated alternately with layers of an oxide in the corundum structure,the oxide in the corundum structure being Cr₂O₃, Fe₂O₃, or a mutualsolid solution of two or more oxides selected from the group consistingof Cr₂O₃, Fe₂O₃ and Al₂O₃ or layers of the oxide in the corundumstructure mixed with the metal of the oxide, and wherein each of thealumina layers mainly in the α-crystal structure has a thickness of 0.5μm or less.
 2. The protective alumina film according to claim 1, whereinthe oxide layer in the corundum structure or the mixed layer of theoxide in the corundum structure and the metal of the oxide has athickness of 2 nm or more.
 3. The protective alumina film according toclaim 1, wherein the total thickness of the oxide layers in the corundumstructure or the mixed layers of the oxide in the corundum structure andthe metal of the oxide is 10% or less with respect to the thickness ofthe protective film.
 4. The protective alumina film according to claim1, wherein a total thickness of the layers of an oxide in the corundumstructure is 10% or less with respect to a thickness of the protectivefilm.
 5. The protective alumina film according to claim 1, wherein eachof the alumina layers mainly in the α-crystal structure has a thicknessof 0.35 μm or less.
 6. The protective alumina film according to claim 1,wherein each of the alumina layers mainly in the α-crystal structure hasa thickness of 0.2 μm or less.
 7. The protective alumina film accordingto claim 1, the oxide in the corundum structure is Fe₂O₃.
 8. A method offorming the protective alumina film according to claim 1 on a basematerial or on an undercoat film previously formed on the base material,comprising forming the alumina layers mainly in the α-crystal structurealternately with the layers of oxide in the corundum structure or thelayers of the oxide in the corundum structure mixed with the metal ofthe oxide during formation of the alumina film.